The present invention relates to improvements in or relating to propellers, now more generally referred to as "impellers", of the type designed for producing a turbulent motion within a gaseous, liquid or other medium, or a medium having a more or less pronounced consistency, in order to effect in such medium the stirring of a mixture, an aeration, a mixing or dispersive action. However, this enumeration should not be construed as limiting the scope of the present invention.
As a rule, the problem to be solved in the technical field concerned is to produce in a closed or open vessel or the like a stirring or turbulent action distributed throughout the vessel in which the impeller is mounted and the medium is to be processed, with the minimum power consumption.
The various research efforts performed up to now with a view to solve this problem have been directed principally to the study of the vessel shapes and also of the impeller blade profiles, with correlative attempts to reduce through the use of suitable techniques the sometimes high cost of the vessel and blades.
Prior researches made by the Applicant proved that substantial power savings could be made when preparing a mixture by using an impeller having the best possible "pumping" (or "blowing") characteristics.
Pumping, which is the fluid flow output passing through the impeller determines the creation, in the medium receiving the impeller, of movements causing both the transport of particles constituting the medium and a distortion of the particles. This distortion, due to differential speeds, is due to the turbulent energy (W.sub.T) created by the impeller, and the transport proper is due to the displacement energy (W.sub.D) also created by the impeller.
The level of turbulent energy required for producing a predetermined effect is actually subordinate to this desired effect. Thus, for instance, it is easy to mix two miscible liquids, but on the other hand it is difficult to create particles of gradually decreasing magnitude in one phase dispersed in another phase.
Generally, the permissible energy savings are achieved by not exceeding the strict minimum amount of turbulent energy W.sub.T which is necessary for obtaining the desired result.
Having thus ascertained the importance of the flow output per unit of power consumption of the impeller, the Applicant directed his search more particularly towards the fluid flow patterns in the mixer vessel. This study eventually proved that a number of advantageous properties could be obtained by improving the knowledge of these flow patterns. As a rule, these improved properties led to a substantial reduction in the power consumption required for obtaining a given local effect through a better distribution of the active areas in the mixing volume, in general.
Observing the phenomena produced in a mixing vessel due to the operation of a conventional propeller proves that, in contrast to what occurs in a indefinite medium (the term "indefinite medium" denotes a liquid area not influenced by solid walls, for example in the case of a ship propeller churning sea water, in opposition to a closed vessel in which the dimensions of the vessel are small in relation to the dimensions of the impeller so that certain reflexion effects occur due to the presence of the walls) wherein the propeller jet is cylindrical, a characteristic outflaring of the jet a is produced, this jet thus assuming the shape of a more or less open cone having an apex angle .alpha. (see FIG. 1 of the attached drawings). This outflaring effect is subordinate to the proximity of the lateral walls and also to the viscosity of the fluid filling the vessel c. The more or less outflared configuration of the jet under given geometrical properties of the vessel and fluid viscosities may constitute an advantage, but in most instances it constitutes an inconvenience, inasmuch as the jet energy is considerably diluted therein and the local effects at points remote from the impeller may drop below a critical limit. Thus, the apex angle .alpha. of the cone formed by the blowing impeller may attain 120.degree. in water if ratio d/D of the impeller diameter to the vessel diameter is 0.7 and the jet bursts out either in the bottom of the said vessel or against its vertical side wall, according to the distance from the impeller to the bottom.
Moreover, the slower the dissipation of the jet energy, the greater the distance attained by the fluid to which energy is impressed by the impeller, the dissipation being due not only to the peripheral friction forces increasing with the external surface and therefore with the outflaring, but also to the internal turbulent effects. These effects depend on the continuity of the impeller profile characteristics.